Waveguide polarizers are phase shifters which receive a linearly polarized signal as input and convert it into a circularly polarized output signal. Waveguide polarizers operate by separating an input signal, E, into two orthogonal electric field signal components, Ex and Ey. One signal component is delayed relative to the other to introduce a phase shift of 90 degrees. To achieve a 90 degree difference, the period of delay is chosen to be one-quarter of the wavelength of the signal in the waveguide at the desired frequency. The combination of the two signal components results in a circularly polarized signal, also known as a rotating linear signal.
Waveguide polarizers typically have rectangular or circular cross sections. A generic rectangular waveguide, where the waveguide walls are aligned with the X and Y axes, is illustrated in FIG. 1. A linearly polarized input signal is aligned so that the signal polarity is from corner to corner of the waveguide entrance. Differences in the electrical properties between the two pairs of opposing walls delays one of the components relative to the other by about 90 degrees to provide a circularly polarized output signal.
Waveguide polarizers are generally used in high frequency applications such as transmitters and receivers for satellite communication, as well as various radar applications. Although it is relatively easy to construct a polarizer which provides an optimum phase difference of 90 degrees between Ex and Ey for a single frequency, it is more difficult to produce a polarizer with a wide bandwidth because the phase delay of a signal component varies according to the wavelength of the input signal.
The theoretical bandwidth of a rectangular waveguide polarizer is limited to frequencies between c/2a.sub.n and c/a.sub.w, where c is the speed of light and a.sub.n and a.sub.w are the width of the waveguide along the narrowest and widest side, respectively. The lower frequency limit is the frequency where signals do not propagate and therefore the waveguide cuts off. The higher frequency limit is the frequency where higher order signal modes begin to propagate in the waveguide, interfering with the dominent/desired mode signal. For optimum circular polarity, the phase difference should be 90 degrees. Reasonably good circular polarization is achieved with a phase difference between about 80 degrees and 100 degrees. This range may be considered to be the usable waveguide bandwidth. Of course, other definitions of good polarization may be used according to the demands of the application.
Various methods have been employed to increase the available bandwidth of polarizers. In one configuration, shown in FIG. 2a, a dielectric slab is introduced inside a circular waveguide. The dimensions and composition of the slab are chosen so that one signal component is delayed relative to the other as required. FIG. 2b is an illustration of a dielectric loaded rectangular waveguide. In this type of waveguide, a different type of dielectric material is applied to each pair of opposing walls. The two different materials provide different phase velocities for the propagating signal components in the waveguide. With the proper selection of dielectric materials, good performance over a broad band can be achieved. However, the required dielectric materials are relatively costly. In addition, it is difficult to repeatably manufacture waveguides of this type which have the same characteristics without fine tuning individual units to achieve the proper performance. Accordingly, waveguide polarizers relying on dielectrics are too expensive to manufacture in large quantities for many commercial applications.
An alternate waveguide configuration is illustrated in FIG. 2c. In this configuration, transverse corrugations or slots are introduced along one wall of the waveguide or on opposing walls. The corrugations may be formed of the same material as the conducting waveguide, such as metal, and function as an artificial dielectric. In the waveguide of FIG. 2c, the propagation velocity of signal components in the corrugated walls will differ from the velocity in the flat walls. By adjusting the geometry of the corrugations appropriately, a phase shift of 90 degrees may be achieved for a limited frequency range. However, while the use of a dielectric is avoided, the transverse nature of the corrugations requires that they be investment cast or machined, production techniques which become significantly more expensive for high volume production when compared to die cast fabrication.
Transverse corrugations on two opposing walls of a rectangular waveguide may be combined with dielectric loading on the other two flat walls as discussed by E. Lier and T. Schaug-Pettersen in A Novel Type of Waveguide Polarizer with large Cross-Polar Bandwidth, IEEE Transactions on Microwave Theory and Techniques, Vol. 23, No. 11, p. 1531-1534, November 1988. The polarizer configuration disclosed by Lier and Schaug-Pettersen has approximately a 40% bandwidth with a 20 dB polarization ratio, or a phase difference of between 78.58 to 101.42 degrees. While this arrangement may provide an increased bandwidth over the waveguide of FIG. 2c, it still is subject to the manufacturing difficulties introduced by the transverse corrugations, in addition to the greater cost and repeatability concerns which result from the use of dielectrics.
Lier and Schaug-Pettersen also note that transverse corrugations have been placed on all four walls of the waveguide in an attempt to increase bandwidth. However, even though the use of a dielectric is avoided in this arrangement, the usable bandwidth is limited when compared with other waveguides, particularly ridged waveguides, discussed below, because the low end cutoff frequency and high order mode propagation frequency are not extended at all by the additional corrugations. In addition, placing corrugations on all four walls compounds the manufacturing difficulties introduced by adding transverse corrugations to only one or two walls.
Yet another polarizer configuration is illustrated in FIGS. 2d and 3. In the waveguide of FIG. 2d, an axial ridge is provided on one wall of a rectangular waveguide (single ridged) or on a pair of opposing walls (dual ridged), while the remaining walls are left blank. As shown in the cross-section of FIG. 3, the added ridges alter the propagation velocity of signal component E1 travelling perpendicular to the ridged walls compared to the component E2 traveling perpendicularly to the flat walls. The characteristics of the waveguide may be determined by adjusting the height (h), width (w), and length (L) of the ridges using techniques well known to those skilled in the art.
Although single and dual ridge polarizers are suitable for mass production using techniques such as die casting, these polarizers have a relatively narrow usable bandwidth because the phase characteristics of the ridged wall(s) differ considerably from that of the adjacent blank walls. Thus, outside of the "center" frequency, where the designed 90 degree phase shift is present, the phase shift curves for the two signal components diverge quickly, resulting in a relatively narrow region where good circular polarization is achieved, i.e., a phase difference between Ex and Ey of, for example, 80 and 100 degrees.
Accordingly, it is an object of the present invention to provide a waveguide polarizer which has a wide operating bandwidth over which good circular polarization is achieved.
It is a further object of the invention to provide a waveguide polarizer which may be inexpensively fabricated using die cast techniques.
Yet another object of the invention is to provide a waveguide polarizer which does not require the use of dielectric materials.